1. Field of the Invention
This invention relates to improvements in thermoacoustic refrigeration, and in particular to a thermoacoustic refrigeration device which employs a gas-vapor mixture as the working fluid.
The invention also relates to a refrigeration method, and in particular to a thermoacoustic refrigeration method that modifies the conventional thermoacoustic refrigeration cycle by adding condensation and vaporization to the thermoacoustic cycle.
In conventional thermoacoustic refrigeration, heat energy is transported primarily or solely by waves acoustically induced in an inert gas. The waves may be standing waves or traveling waves that oscillate or travel from one side of the thermal stack to the other, but in either case are arranged to exploit the temperature differential between areas of compression and areas of rarefaction relative to a thermal stack or regenerator, the thermal stack transferring heat energy from the cold areas to the hot areas to achieve refrigeration. The purpose of the vaporization-condensation cycle of the invention is to increase the efficiency of heat transport by harnessing the translational motion of the vapor, as well as the usual acoustic oscillations, to transport the heat energy from one end of the thermal stack to the other, i.e., by using acoustic mass transfer as well as acoustic heat transfer to transport heat energy up the heat-absorbing stack.
2. Description of Related Art
The basic principles of thermoacoustic refrigeration have been known for more than a decade. In its most basic form, thermoacoustic refrigeration is a process that utilizes acoustic energy to pump or transport heat through a thermal stack between a cold heat exchanger at one end and a hot heat exchanger at the other end, the acoustic energy being in the form of standing or traveling acoustic waves generated by a loudspeaker or similar moving part to cause the mechanical compression and expansion of a working fluid needed for the cooling cycle.
In all such devices, heat is transported solely by acoustic waves in an inert gas. Since the acoustic wave generator is the only moving part in a thermoacoustic refrigeration device, thermoacoustic refrigeration devices have the potential for greater reliability, smaller size, and lower cost than conventional refrigeration devices. Despite these potential advantages, however, thermoacoustic refrigeration has one substantial disadvantage that has heretofore prevented more widespread application of the technology, namely relatively low cooling power in comparison with conventional vapor cycle refrigeration devices. Essentially, the problem is that the oscillations in temperature resulting from application of an acoustic wave to the inert gas used as a working fluid are relatively small and thus have a limited ability to transfer heat.
The basic standing wave version of the conventional thermoacoustic device, illustrated in FIG. 1, is simply a hollow resonance tube 1 filled with an ordinary inert gas and having a speaker or acoustic driver 2 at one end, a hard termination 3 at the other end, and a dry stack or regenerator 4 sandwiched between hot and cold heat exchangers 5,6. The dry stack or regenerator is composed of a solid material and finely divided into sections or passages with which the working fluid exchanges heat with the solid material. In the case of a standing wave, a temperature differential is established between areas of compression, in which heat is transferred from the working fluid to the thermal stack, and rarefaction, in which heat is transferred from the thermal stack to the working fluid.
Details of the refrigeration device shown in FIG. 1 may be found in T. Hofler, xe2x80x9cThermoacoustic refrigeration design and performance,xe2x80x9d Ph.D. dissertation, Physics Department, University of California at San Diego, 1986, while additional information and background on standing or traveling wave refrigeration devices may be found in U.S. Pat. Nos. 4,114,380 (Ceperley); 43,555,517 (Ceperley); 4,398,398 (Wheatley et al.); 4,489,553 (Wheatley et al.); 4,722,201 (Hofler et al.); 4,858,441 (Wheatley et al.); 4,953,366 (Swift et al.); 5,165,243 (Bennett); 5,647,216 (Garrett); 5,673,561 (Moss); 5,901,556 (Hofler); 6,032,464 (Swift et al.); 6,164,073 (Swift et al.); and 6,233,946 (Matsuda).
The present invention, in contrast, augments the purely acoustic heat transfer of previously proposed thermoacoustic refrigeration devices by adding an evaporation-condensation cycle similar to that of conventional vapor-cycle based refrigeration devices, but without the mechanical complexity of the conventional refrigeration device. This is achieved by mixing vapor with an appropriate gas working fluid, and by permitting the working fluid to locally evaporate and condense on the hot and cold ends or sides of the thermal stack, thereby transferring heat energy by acoustic mass transfer as well as by acoustic heat transfer.
With the proper selection of the gas-vapor mixtures used as working fluids in the thermoacoustic refrigeration devices of the invention, criteria for which are described below, predicted heat pumping power and coefficient of performance relative to the Carnot cycle can be increased for an inert gas-vapor working fluid compared to a similar purely inert gas working fluid.
It is accordingly an objective of the invention to provide a thermoacoustic refrigeration device that provides improved performance without a substantial increase in complexity.
It is also an objective of the invention to provide a thermoacoustic refrigeration method that increases the heat transfer capability of the thermoacoustic refrigeration device by modifying the conventional thermoacoustic refrigeration cycle to include evaporation and condensation, thereby enlisting translation of vapors as a mechanism for heat transfer in addition to the conventional acoustic heat transfer.
It is a still further objective of the invention to provide criteria for selecting a gas-vapor mixture that can be used in a thermoacoustic refrigeration device in order to increase performance by making use of translating vapor to carry heat energy, and condensation of the vapor to transfer the heat energy to the refrigeration stack.
According to a preferred embodiment of the invention, the thermoacoustic refrigeration device of the invention may include, as in a conventional thermoacoustic refrigeration device, a resonance tube filled with a working fluid, an acoustic driver at one end, a hard termination at the other end, a thermal stack situated within the tube and made up of a finely divided structure composed of a solid material, and cold and hot heat exchangers at opposite ends of the stack. Unlike the inert gas of the conventional device, however, the working fluid employed by the refrigeration device of the preferred embodiment is a gas-vapor mixture, the stack is composed of a solid material that is wettable by condensed vapor, and means are provided to return condensed vapor from the cold side of the stack to the hot side. The return means may be a pump or, for simplicity, a wick. Alternatively, the stack may be arranged to permit return solely or primarily by gravity.
The method of the preferred embodiment of the invention modifies conventional thermoacoustic cooling, which simply involves generating acoustic waves in the working fluid, by modifying the thermoacoustic refrigeration cycle to include at least the following steps:
a. In response to an applied acoustic wave, a parcel of gas in the working fluid is caused to undergo translation along the stack and consequent acoustic compression, thereby decreasing the parcel""s volume and increasing its temperature;
b. The decreased volume and increased temperature increases the partial pressure of the vapor within the parcel;
c. The parcel then slows, stops, and reverses its translational motion, while at the same time exchanging heat and vapor with the stack as a result of the parcel""s increased temperature relative to the stack;
d. At the time of reversal, the increased partial pressure relative to the vapor pressure at the stack wall causes vapor to condense from the parcel to the stack plate;
e. The gas parcel then undergoes acoustic rarefaction and is translated back past the ambient position, increasing its volume and decreasing its temperature;
f. The acoustic rarefaction in turn causes a decrease in partial pressure of vapor within the parcel;
g. The parcel again slows, stops and reverses its translational motion while exchanging heat and vapor with the stack, this time absorbing heat from the stack;
h. Since the partial pressure of the vapor in the parcel is lower than the vapor pressure at the stack wall, the vapor will evaporate to the parcel from the liquid layer coating the stack.
Because of the modifications to the conventional thermoacoustic refrigeration cycle, heat is transported from one end of the stack to the other, both as a result of the temperature differentials resulting from compression and rarefaction of the gas, and as a result of vapor moving up the stack and exchanging heat at each end.
The performance of the refrigeration device of the preferred embodiment depends on the characteristics of the gas-vapor mixture. According to another aspect of the invention, the gas-vapor mixture is chosen to:
a.) maximize the amount of heat carried by mass relative to the amount of heat carried by thermal effects, represented by xcex5D defined by the relationship:             ϵ      D        =                  1                  c          p                    ⁢                        ρ          2          o                          ρ          o                    ⁢                        n          o                          n          1          o                    ⁢              γ                  γ          -          1                    ⁢              (                              s            mix                    -                      s            liquid                          )              ,
where xcfx812o is the mass density of the vapor in the mixture, xcfx81o is the mass density of the mixture, no is the number density of the mixture, n1o is the number density of the inert gas, smix is the entropy per unit mass of the mixture Sliquid the entropy per unit mass of the condensed liquid, and cp is the heat capacity at constant pressure per unit mass of the mixture and,
b.) minimize the heat transfer coefficient W defined by the relationship:       ϕ    =                            γ          -          1                γ            ⁢              l                              R            o                    ⁢                      T            o                                ,
where xcex3 is the ratio of the specific heats of the gas and of the mixture; 1 is the latent heat of vaporization of the mixture per mole, To is the ambient temperature, and Ro is the universal gas constant. This parameter controls the critical gradient and therefore the stack length necessary to produce a given temperature difference.
Although a particular standing wave device is illustrated herein, those skilled in the art will appreciate that the device and the method of the invention may also be applied to traveling wave refrigeration techniques, and that the simple tube, stack, and heat exchange structure of the illustrated embodiment may be freely varied by those skilled in the art in accordance with the principles described in any of the above references, without departing from the scope of the invention, so long as the modified device employs a gas-vapor mixture as the working fluid.